Reduced frequency of blocks for concurrent gc

Immediately return for references that are marked before reference
processing without blocking. Soft references are kept in the queue until
the reference processor stops preserving, after which, all marked
references are removed. Finalizer references will still block on get().

Bug: 15471830
Change-Id: I588fcaef40b79ed7c95a4aa7f4fc2e17ee0c288f

1 files changed, 15 insertions, 10 deletions

diff --git a/runtime/gc/reference_processor.cc b/runtime/gc/reference_processor.cc
index a58df8ec2c..7988af7f6b 100644
--- a/runtime/gc/reference_processor.cc
+++ b/runtime/gc/reference_processor.cc
@@ -61,15 +61,20 @@ mirror::Object* ReferenceProcessor::GetReferent(Thread* self, mirror::Reference*
     }
     // Try to see if the referent is already marked by using the is_marked_callback. We can return
     // it to the mutator as long as the GC is not preserving references. If the GC is
-    // preserving references, the mutator could take a white field and move it somewhere else
-    // in the heap causing corruption since this field would get swept.
     IsMarkedCallback* const is_marked_callback = process_references_args_.is_marked_callback_;
-    if (!preserving_references_ && is_marked_callback != nullptr) {
+    if (LIKELY(is_marked_callback != nullptr)) {
       mirror::Object* const obj = is_marked_callback(referent, process_references_args_.arg_);
       // If it's null it means not marked, but it could become marked if the referent is reachable
-      // by finalizer referents. So we can not return in this case and must block.
+      // by finalizer referents. So we can not return in this case and must block. Otherwise, we
+      // can return it to the mutator as long as the GC is not preserving references, in which
+      // case only black nodes can be safely returned. If the GC is preserving references, the
+      // mutator could take a white field from a grey or white node and move it somewhere else
+      // in the heap causing corruption since this field would get swept.
       if (obj != nullptr) {
-        return obj;
+        if (!preserving_references_ ||
+           (LIKELY(!reference->IsFinalizerReferenceInstance()) && !reference->IsEnqueued())) {
+          return obj;
+        }
       }
     }
     condition_.WaitHoldingLocks(self);
@@ -113,14 +118,14 @@ void ReferenceProcessor::ProcessReferences(bool concurrent, TimingLogger* timing
   timings->StartSplit(concurrent ? "ProcessReferences" : "(Paused)ProcessReferences");
   // Unless required to clear soft references with white references, preserve some white referents.
   if (!clear_soft_references) {
-    TimingLogger::ScopedSplit split(concurrent ? "PreserveSomeSoftReferences" :
-        "(Paused)PreserveSomeSoftReferences", timings);
+    TimingLogger::ScopedSplit split(concurrent ? "ForwardSoftReferences" :
+        "(Paused)ForwardSoftReferences", timings);
     if (concurrent) {
       StartPreservingReferences(self);
     }
-    // References with a marked referent are removed from the list.
-    soft_reference_queue_.PreserveSomeSoftReferences(&PreserveSoftReferenceCallback,
-                                                     &process_references_args_);
+
+    soft_reference_queue_.ForwardSoftReferences(&PreserveSoftReferenceCallback,
+                                                &process_references_args_);
     process_mark_stack_callback(arg);
     if (concurrent) {
       StopPreservingReferences(self);